In recent years, a number of deep or ultra-deep carbonate oil and gas reservoirs have been developed in succession in China. Such oil and gas reservoirs usually can't provide seepage channels required for economical exploitation and most of them have no natural productivity owing to their poor physical properties and low porosity and permeability. Therefore, all of them require reservoir stimulation with acid fracturing and acidification measures, etc.
Conventional acid fluids have drawbacks such as low viscosity, small penetration depth, high filtration rate, and severe corrosion, etc.; therefore, often the acidification improvement can't attain the expected result. As a sort of retarded acid, viscous acids can attain a good effect for medium-permeability, high-permeability, or even low-permeability reservoirs that have fractures. By adding a high-performance viscosifier into an acid fluid, the viscosity of the acid fluid can be increased, the mass transfer velocity of hydrogen ions to rock wall surface can be decreased, and thereby the fluid loss of the acid fluid and the acid-rock reaction rate can be improved, the effective action distance of the acid fluid can be increased, and the reservoir stimulation effect can be improved remarkably. Developing a high-performance acid fluid viscosifier is a subject that has been explored continuously by oil production workers.
At present, common acid fluid viscosifiers mainly include natural polymer acid fluid viscosifiers, synthetic polymer acid fluid viscosifiers, and viscoelastic surfactant acid fluid viscosifiers, etc.
Natural polymer acid fluid viscosifiers, such as guanidine viscosifiers and their derivatives, cellulose viscosifiers and their derivatives, etc., have to be used in combination with several auxiliary agents, and have to be used under the conditions of low temperature and low shearing rate, because they are susceptible to degradation by fungi and biological enzymes. Therefore, they are seldom used nowadays, and are rarely seen among the product varieties developed lately.
As an example of viscoelastic surfactant acid fluid viscosifier, the polymer-free viscous acid for acid fracturing of carbonate oil reservoirs disclosed in the patent document CN102453480A consists of the following components (in weight percentage): hexadecyl trimethyl ammonium bromide: 5%-0.7%; 36-38% hydrochloric acid: 20-25%; Fe stabilizer: citric acid (0.2-0.25%), CX-301 (0.05-0.1%), or TW-1 (0.3-0.5%); quaternary ammonium bis-imidazoline: 0.22-0.25%; water: the remaining content. Though the surfactant acid fluid viscosifiers don't involve problems such as residue and solution time, they are expensive, and some of the components are toxic. Therefore, they are unsuitable for use widely.
The Chinese patent document CN105907386A has disclosed an emulsion viscous acid, which comprises an emulsion viscosifier, an inhibitor, a Fe stabilizer, a cleanup additive, hydrochloric acid, and water, wherein, the volumetric fraction of the emulsion viscosifier is 1%-5%, the volumetric fraction of the inhibitor is 0.5%-1.5%, the volumetric fraction of the Fe stabilizer is 10%-25%, the volumetric fraction of the cleanup additive is 0.5%-1.5%, the volumetric fraction of hydrochloric acid is 10%-25%, and water accounts for the remaining content. The emulsion viscosifier can save energy required for powder production, can be dissolved more easily in the acid fluid, and can improve the application effect. However, it can't meet the requirement of construction at remote sites, because of its high transportation cost.
The patent document CN102994070A has disclosed a preparation process of a polymeric acid viscosifier. The process comprises the following steps: (a) dissolving polyethylene, pyrrolidone, and a crosslinker in a solution, and loading the solution into a reactor in which an organic solvent and an initiator exist; (b) heating up to a specific temperature and holding at the temperature; (c) heating up further to 130° C., and holding at the temperature for 8 h for reaction, so as to prepare a polymer; (d) treating by precipitation, filtering and drying, to obtain a powder product. Though the products is convenient to transport and simple in process, but the reaction temperature in the preparation process is too high and adverse to large-scale industrial production; moreover, the introduction of the crosslinker may cause increased dissolution time of the product in the acid fluid, and has adverse impact on field use.
As the understanding on viscosifiers is deepened and the field application of viscous acid techniques is widened, the requirements for acid viscosifiers become higher and higher, mainly including high viscosifying ability of the viscosifier for acid fluids, high stability, and simple field application, etc.